Electric arc furnaces have been used for a long time, and are increasingly used, for the production of steel by melting scrap iron or other ferrous raw material, for example, pre-reduced ore.
In general, an electric arc furnace comprises a melting chamber having a bowl-shaped bottom with a refractory lining and a lateral wall, with its upper portions closed by a removable arched roof, the chamber being associated with one or more electrodes connected to a power source. Each electrode is fixed to the end of an arm overhanging the chamber, and penetrates vertically into the chamber through an opening in the roof provided for this purpose.
In general, the furnace is supplied with a.c. power by means of several consumable electrodes. For some time, however, even for high capacities, d.c. furnaces have been used, and this offers certain advantages. In this case, one or more consumable electrodes are used, generally connected to the negative pole of the power source, and one or more fixed electrodes forming an anode, placed in the hearth, and connected to the positive pole of the power source, by one or more return conductors.
Such a furnace is, moreover, associated with a certain number of ancillaries such as electrical installations, means for charging scrap iron and additives, means for removing the molten steel and slag, flue gas removal and purification circuits etc.
Furthermore, to reduce the energy consumption, such an installation also generally comprises means of preheating the scrap iron before it enters the furnace.
This rather complex set of costly and bulky equipment is placed in a building which generally comprises a scrap iron reception and rehandling zone, a preheating zone and a preparation zone.
The raw scrap iron arrives in the reception zone in one or more transport containers or "baskets", which are picked up and transported from one zone to another by one or more overhead travelling cranes running above the entire installation.
Each basket is generally provided with an opening bottom for dumping the scrap into the chamber of the furnace whose roof has been removed. For this purpose, the arch making up the roof and the electrodes can be suspended from a horizontally-travelling gantry or, preferably, from arms mounted and rotating about a vertical shaft on a base placed next to the furnace.
During the melting and refining phases, the production of the steel generates a large volume of hot and dusty gases which must be purified before being discharged to the exterior. The arch closing the furnace is therefore provided with one or more openings connected to a purification circuit which comprises dust removal means.
The heat contained in these flue gases is usually recovered by using them to preheat the scrap. While awaiting charging, each new charge of scrap is therefore placed in a preheating chamber or cell placed next to the furnace and connected to the flue gas circuit of the furnace, which thus transfers their heat to the scrap placed in the chamber.
Before the flue gases enter the preheating chamber, they are often sent to a combustion chamber for burning off the carbon monoxide or other unburnts, and recovering a portion of the dust entrained by the flue gases.
The flue gas removal circuit of the furnace is therefore rather complex and bulky, and could be the source of accidental pollution.
Each basket containing a charge of cold scrap and positioned in the reception zone is picked up by the overhead travelling crane for transport to a preheating cell, where it remains for the necessary time. After preheating, the basket is transported by the overhead travelling crane above the chamber of the furnace into which the preheated scrap is dumped by opening the bottom.
Obviously, during charging, the arch of the furnace must be opened, and this causes a strong emission of flue gases. Steel and slag may also be projected when the scrap is dumped into the chamber.
To simplify the handling operations and to reduce the pollution, and, at the same time, to improve the thermal and energy efficiency, it was proposed some years ago to use a preheating chamber equipped with an opening bottom and with a removable cover, and capable of travelling between a preheating position next to the furnace and a position for dumping the scrap into the furnace. In the preheating position, the cover is opened, and a scrap charge is introduced into the chamber. The chamber is then closed and connected to the circuit evacuating the flue gases from the furnace. These hot gases pass through the chamber, transferring a portion of their heat to the scrap, and are removed by a duct to the purification circuit.
After preheating, the preheating chamber is positioned above the furnace to dump the preheated scrap by opening the bottom.
Applicant's EP 0514.526 dicloses, an installation of this type improved in order to simplify the gas circuits and to reduce the risks of pollution.
However, it has been found that, while preheating the scrap helps to achieve some energy conservation, it may also present drawbacks. In fact, depending on the origin of the ferrous materials, and especially of the scrap iron used to produce the steel, harmful compounds, difficult to eliminate, are liable to be produced in the warming phase. This happens particularly in the case in particular of commercial scrap, which is often polluted by organic substances such as plastics, oils and paints, which generate organo-volatile compounds and particularly organo-chlorinated compounds, some of which, by decomposing, can produce toxic substances such as dioxins and furans.
To prevent such pollution, it is necessary either to sort the scrap iron, or to subject it to pretreatment. These operations obviously increase the cost of the scrap.